The bottom line is, what can potentially happen to you if you withhold an intervention? This goes for any condition, not just the current SMR debate. Being struck on the mouth with a closed fist, without any other insults, shouldn't hold potential for a C-spine injury. If the pt did a face plant afterwards, or hit the back of their head on a curb, then restiction would be warranted, even if the potential for true spinal injury is quite low.
If you're not clear on what situations would necessitate c-spine precautions, than consult your OMD for a definitive answer.
If you're not clear on what situations would necessitate c-spine precautions, than consult your OMD for a definitive answer.